Characterization of In-Situ Floc Sizes Over the Vertical Within the Mississippi River and its Distributaries

Abstract:

The Mississippi River transports large amounts of sand and mud. While models can reasonably predict the long-term transport of sand, predicting the movement of mud is more difficult. The difficulty is largely a result of the cohesive properties of silts and clays that, when conditions are right, can combine to form aggregates known as flocs. Gaining a better understanding of the influences on floc sizes in the freshwater environment is vital to improving the modeling of mud transport. The fragile nature of flocculated sediment necessitates the use of in-situ instrumentation to minimize floc disruption from the natural turbulent river conditions. To accomplish this, we packaged a camera, microscope objective, and light source within a waterproof housing to allow for high resolution in-situ imaging of sediment particles down to 10 microns. A field campaign of the Mississippi River Delta was conducted in late June of 2020, during which images were collected over the vertical in the main channel at the Bonnet Carre Spillway, the main channel above Venice, and in six of the main distributaries including: Baptiste Collette, Cubits Gap, Pass-a-Loutre, South Pass, Southwest Pass, and the West Bay Diversion. Images collected during the campaign revealed d₅₀ freshwater floc sizes typically between 70 to 130 microns, with size varying over the vertical. The observation of freshwater flocculation of sediment has major implications for the potential of retaining mud through land-building sediment diversion projects.